The present invention relates to an optical unit having optical waveguides, particularly an optical unit having a so-called MMI (Multi-Mode Interference) type optical waveguide. According to the present invention it is possible to provide a wavelength multiplexer of an extremely good quality.
With the recent development of multi-media communication, including the Internet, studies are becoming more and more extensive about WDM (Wavelength Division Multiplexing) technique which is intended to attain a high-speed large-capacity communication. As one of optical parts which are important for building a WDM communication system there is known a wavelength multiplexer which combines or branches wavelengths of light having plural wavelengths. Above all, what is attracting attention of many concerns from the standpoint of attaining the reduction of cost and size and a high function is a method wherein optical waveguides using quartz (glass) or a polymer for example and a wavelength multiplexer are formed on a substrate and optical transmitter and receiver are mounted thereon to achieve integration.
As the wavelength multiplexer there is known, for example, a directional coupler type or a Mach-Zehnder interferometer type. Further, in connection with the technique advantageous to the reduction in size of a module, there is known, for example, such a technique as is disclosed in Japanese Patent Laid Open No. Hei 8-190026 (Article 1).
FIG. 1 shows a filter type wavelength multiplexer described in the above Article 1. In this optical wavelength multiplexer, linear optical waveguides 1 and 2 are crossed and an optical filter 4 is disposed in a cross point of the two. In the illustrated example, a WDM signal is divided into reflected light and transmitted light by utilizing wavelength transmitting and reflecting characteristics of the optical filter 4. According to this structure it is necessary to make design in such a manner that an intersecting point 3 of the axes of the two optical waveguides 1 and 2 which intersect each other at an angle of 2xcex8 lies on an equivalent reflection center plane 5 of the optical filter 4. In FIG. 1, central axes of the optical waveguides 1, and 2 are indicated at 6, and 7 respectively.
It is the first object of the present invention to enlarge the tolerance for a deviation of an installed position of reflecting means typical of which is an optical filter installed within an optical guide in an optical unit utilizing light reflected by the reflecting means, the optical unit being typified by a wavelength multiplexer. In other words, it is the first object of the invention to minimize the increase of loss in the optical unit based on a deviation of the installed position of the reflecting means.
It is the second object of the present invention to diminish an optical crosstalk in the above optical unit, especially a wavelength multiplexer.
A typical form for achieving the above first object of the invention can take the form of, for example, a wavelength multiplexer which is formed on a plane substrate and which functions to combine or branch wavelengths of a signal light having plural wavelengths. According to the present invention it is possible to achieve the above first and second objects together.
A typical mode of the present invention resides in an optical unit having optical waveguides, including at least first, second and third optical waveguides, a fourth optical waveguide capable of propagating light in a multi-mode, and an optical filter disposed perpendicularly to a traveling direction of light in the fourth optical waveguide, the first optical waveguide being connected to a first end face of the fourth optical waveguide, the second and third optical waveguides being connected to predetermined individual positions of a second end face opposed to the first end face of the fourth optical waveguide, the first and second end faces of the fourth optical waveguide being end faces intersecting the traveling direction of light in the fourth optical waveguide, and the fourth optical waveguide being an optical waveguide capable of propagating light in a multi-mode such that upon input of light having a first wavelength from either the second or the third optical waveguide, light corresponding to the light input of the first wavelength can be propagated into the first optical waveguide after passing through the optical filter by the propagation of light in the fourth optical waveguide, and upon input of light having a second wavelength from either the second or the third optical waveguide, light corresponding to the light input of the second wavelength can be propagated into a light input-free optical waveguide out of the second and third optical waveguides through reflection by the optical filter.
The principle of the present invention will now be described. FIG. 7 is schematically illustrates in what manner light of a wavelength passing through a filter 15 in wavelength multiplexer according to the present invention is incident from a waveguide 12 (distance from a center line: D), then is propagated through a multi-mode interference type waveguide 10 and is emitted to a waveguide 11. In the multi-mode interference type waveguide 10, for example, a signal light incident from the second optical waveguide 12 excites various modes of lights in the multi-mode interference type optical waveguide 10. Although in FIG. 7 show excited states of zero-, first- and second-order modes, there may be excited higher modes. Since the propagation velocities of the excited modes are different from one another, there occur phase differences from one another with propagation through the multi-mode interference type waveguide (as shown FIG. 7), and a light intensity distribution, which is the sum of the modes, changes as light is propagated through the multi-mode interference type waveguide. This change is periodical and it is known that at a cycle Lp light intensity distribution is reproduced again into the same shape as an input section (self-imaging of the multi-mode waveguide). The light intensity distribution at the position of Lp/2 which is half of the said period corresponds to a reflected image obtained by folding back the light intensity distribution of the input section symmetrically with respect to the center line of the multi-mode optical waveguide. In the present invention, the length (L) of the multi-mode waveguide is set at the above Lp/2 and the center of the first optical waveguide 11 is placed at the center of peak (i.e., the position of distance D from the center line) of a symmetric optical intensity distribution which occurs at an outlet of the multi-mode optical waveguide, whereby the incident light from the second optical waveguide can be guided again through the optical waveguide 11 with scarcely any loss.
Also as to the case where light of a wavelength reflected by the filter 15 is incident on the above wavelength multiplexer, the same argument as above also applies except that the light is reflected by the filter 15. More particularly, if the filter is installed at a position of L/2=(Lp/2)/2 from the inlet, the light incident from the waveguide 12 is reflected by the filter 15 and thereafter reproduces, at the inlet of the multi-mode waveguide, a light intensity distribution which is symmetric with respect to the center line in comparison with that obtained at the time of incidence. Therefore, if a waveguide 13 is installed at a position (the position of distance D from the center line) symmetric with the waveguide 12 relative to the center line, the light incident from the optical waveguide 12 can be guided again through the optical waveguide 13 with little loss.
In this case, part of the reflected light is propagated reverse through the optical waveguide 12, creating a reflected return light (resulting in poor optical directivity). However, by arranging the optical waveguides 12 and 13 so that the spacing D between both waveguides takes a sufficiently large value, the reflected return light can be sufficiently diminished (resulting in good optical directivity).
In the wavelength multiplexer or optical waveguide device according to the present invention, as in this example, the length and width of the foregoing multi-mode interference type optical waveguide and the connections between the multi-mode interference optical waveguide and the first to third optical waveguides are adjusted so that, after signal lights incident from one or more of the first to third optical waveguides are propagated each as a multi-mode light through the multi-mode interference type optical waveguide, a signal light having a predetermined wavelength is coupled to one or more optical waveguides out of the first to third optical waveguides with low loss.
In the wavelength multiplexer or optical waveguide device according to the present invention, it is preferable that the length L of the above multi-mode interference type waveguide be in the range of 1 to 5 mm and the width thereof in the range of 25 to 70 xcexcm.
In the wavelength multiplexer or optical waveguide device according to the present invention, moreover, the fourth optical waveguide is connected to the multi-mode interference type optical waveguide on the first optical waveguide side. Alternatively, plural optical waveguides other than the first to fourth optical waveguides may be connected to the multi-mode interference type optical waveguide.
Or, the wavelength multiplexer or the optical waveguide device may be in the form of an optical trensmitter module, an optical receiver module, an optical transceiver module, or an optical multiplexer module, characterized in that a light emitting element, a light receiving element, an optical fiber, or means for connection with an optical fiber is provided at end faces of all or part of the first to fourth optical waveguides.
Further, for solving the problem of optical crosstalk as the foregoing second problem, it is useful to adopt the following measure.
Firstly, the wavelength multiplexer or optical waveguide device according to the present invention is characterized in that extension lines of the optical axes of the second and third optical waveguides have an intersecting point outside the multi-mode interference type waveguide or are parallel to each other. An optical transciever module using such a wavelength multiplexer or optical waveguide device comprises, in addition to the wavelength multiplexer or the optical waveguide device, a light receiving element, an optical fiber, or an optical fiber connecting means, which is provided as a light receiving means at both or one end face of the first or the fourth optical waveguide, and a light emitting element, an optical fiber, or an optical fiber connecting means, which is provided as a light transmitting means at both or one end face of the second or third optical waveguide. In this optical transceiver module, the foregoing optical crosstalk is well blocked since the optical filter is optimized to reflect a light which is propagated through the second and third optical waveguides and which has an incident angle close to 0xc2x0.
Secondly, the optical crosstalk has a distribution centered on a direction perpendicular to an end face of the transmission means, but since the optical waveguides used in the present invention can be bent in an arbitrary direction, the optical crosstalk can be made less influential by bending both or one of the first or the fourth optical waveguide in a direction away from the center of the aforesaid optical crosstalk distribution.
Further, the wavelength multiplexer or the optical waveguide device may be in the form of an optical waveguide module wherein it is combined with an optical element such as a light emitting or receiving element, an optical switch, an optical filter, an optical amplifier, or an optical modulator.
Or the optical waveguide module according to the present invention may use a plurality of the wavelength multiplexeres or optical waveguide devices in combination with plural optical elements such as light emitting or receiving elements, optical switches, optical filters, optical amplifiers, or optical modulators to process a plurality of signals at a time or process a signal light having plural wavelengths in plural steps.
Or these optical modules may each be combined with an electric signal processing means such as an integrated circuit or a preamplifier to afford an optical communication module.